1. Field of the Invention
The invention relates to an electric power steering system that assists a vehicle steering operation.
2. Description of the Related Art
A conventional electric power steering system of this type is described in Japanese Patent Application Publication No. 2011-203091 (JP 2011-203091 A). The electric power steering system includes a torque sensor and a controller that controls driving of a motor. The torque sensor includes a Hall IC and a magnetic circuit that applies magnetic flux corresponding to steering torque applied by a driver to the Hall IC. In the electric power steering system, when the steering torque varies in response to a driver's steering operation, the magnetic flux applied from the magnetic circuit to the Hall IC of the torque sensor varies. Thus, a detection signal corresponding to the steering torque is output from the torque sensor. The controller computes the steering torque on the basis of the detection signal from the torque sensor, and computes a current command value on the basis of the detected steering torque value. Then, the controller executes current feedback control for causing an actual current value, which is a value of electric current that is actually supplied to the motor, to follow the current command value to drive the motor, thereby applying assist force to a steering mechanism. In this way, the driver's steering operation is assisted.
The electric power steering system described in JP 2011-203091 A includes a magnetic field generator that periodically applies a magnetic field to the torque sensor, aside from the magnetic circuit. When a magnetic field is applied to the torque sensor through the use of the magnetic field generator, the controller determines whether a detection signal based on the applied magnetic field is output from the torque sensor. When the detection signal based on the applied magnetic field is not output from the torque sensor, the controller determines that a malfunction has occurred in the torque sensor. The controller detects the steering torque on the basis of the detection signal from the torque sensor immediately before the magnetic field is applied to the torque sensor from the magnetic field generator, and the controller continues the current feedback control while holding the detected steering torque value during a period in which the magnetic field is generated from the magnetic field generator.
In the electric power steering system described in JP 2011-203091 A, the current command value is nearly unchanged during a period in which the detected steering torque value is held. If the actual current value of the motor follows the current command value with a high responsiveness, the assist force applied to the steering mechanism is less likely to be changed, and the driver feels a fixed feeling (rigid feeling) in a steering operation. This may be a factor of an uncomfortable feeling given to the driver.